When a machining pulse voltage is repeatedly applied across the discharge machining gap in diesinking or wire-cut electric discharge machining so as to machine a workpiece, particularly a cemented carbide workpiece, the application of normal machining pulses (i.e. pulses between a positively charged workpiece and a negatively charged machining electrode) is found to impede deionization at the machining gap increasingly as the machining progresses. As a result, the resistivity of the machining fluid becomes extremely low locally, especially at the center of the bottom surface of the diesinking electrode and other portions where chip discharge is inadequate.
When this state occurs, a large electrolytic current flows between the workpiece and the machining electrode through the low resistivity region. This produces irregularities on the surface of the workpiece and causes other problems. Moreover, electrolytic substances such as cobalt that electrolyze easily are melted out of the workpiece and form a surface layer (an "affected layer") differing in composition from the underlying workpiece material. Formation of an affected layer on a die being machined shortens the service life of the die and degrades the quality of products formed using the die.
Further, in certain types of diesinking electric discharge machines that use a water-based machining fluid containing a polymer, tar tends to form at the electric discharge machining gap. Since this tar is electrically conductive and locally reduces the impedance at the gap, an electrolytic or arc current arises at the low-impedance region and rapidly produces an oxide deposit. Since the adhering oxide material is an electrical insulator, it markedly lowers the machining speed and deteriorates the nature of the machined surface.
Another problem in this connection is that when tar forms in the electric discharge machining gap, the machining speed falls to such a very low level during finish machining that it may become impossible to continue the finish machining.
Tar also forms in the electric discharge machining gap when a kerosene type machining fluid is used. Since this hinders deionization at the gap, arc discharge is apt to occur. When this happens, the workpiece may suffer surface cracks and the like which greatly shorten the service life of the die being machined.
As it is known that the tar forming at the electric discharge machining gap forms on the negative potential side, a well-known technique has been developed in which the polarity of the machining pulse voltage applied is changed for preventing tar adhesion and removing adhered tar at the gap.
For example, Japanese Patent Application Public Disclosure No. Sho 59-152017(152017/84) includes a passage reading: "A conventional machining power supply for supplying positive pulses for making the workpiece side the anode and negative pulses for making the workpiece side the cathode was tested. However, since the power supply that was used supplied positive and negative pulses by turns or at a ratio such as 3 to 2, the machining speed decreased and it became impossible to control electrode consumption in the manner desired."
For overcoming these problems, the aforesaid Japanese Patent Application Public Disclosure No. Sho 59-152017(152017/84) teaches an electric discharge machining apparatus having a detector for detecting the electric discharge current flow across the machining gap and a switching means for switching the machining power supply so as to supply positive pulses to the gap during periods the detector detects a prescribed electric discharge current and to supply negative pulses to the gap during periods the detector does not detect the prescribed electric discharge current.
Even with this arrangement, however, electrode consumption is high and the machining speed is slow during the supply of positive pulses, particularly, in the non-electrode-consuming region.
Also, in cases where the machining electrode and the workpiece are made of different materials, e.g. where the electrode is made of copper and the workpiece of steel, the normal machining voltage in normal polarity is generally about 20 V and does not include a high-frequency component, whereas the normal machining voltage in reverse-polarity is 25-30 V and includes a high-frequency component. Therefore, when abnormal discharge is detected using the reverse-polarity machining voltage as a reference for discriminating whether or not the machining voltage level is lower than the normal machining voltage, the normal polarity machining is constantly detected to be in a state of abnormal discharge. As a result, adaptive control is implemented even under normal operation, thus unnecessarily slowing the machining speed.
Further, the fact that the machining pulse voltage can be either positive or negative makes it difficult to determine the level thereof by direct detection of the voltage between the electrodes.
One object of the invention is therefore to provide an improved method for conducting electric discharge machining and an apparatus for implementing the method which overcome the aforesaid drawbacks of the prior art.
Another object of the invention is to provide a novel method and a novel apparatus for conducting electric discharge machining which prevent formation of unneeded tar components and reliably preclude the formation of an affected layer on the workpiece.
Another object of the invention is to provide an electric discharge machining method and an electric discharge machining apparatus which suppress the formation of unneeded tar components and minimize machining electrode consumption, without reducing the electric discharge machining speed.
Another object of the invention is to provide an electric discharge machining apparatus equipped with a polarity switching means capable of switching the polarity of a machining pulse voltage for electric discharge machining supplied to an electric discharge machining gap in a highly appropriate manner.